1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly, it relates to an improvement for a short reverse recovery time in a power switching device, such as a power MOSFET or a shorted collector IGBT (Insulated Gate Bipolar Transistor), which is employed in an inductive load circuit such as an inverter circuit for driving a motor.
2. Description of the Background Art
FIG. 1 is a circuit diagram showing a conventional half-bridge circuit employing a power MOSFET, for driving a motor. Referring to FIG. 1, power MOSFETs 1 and 2, and power MOSFETs 3 and 4 are in totem-pole-connection with each other between positive and negative power supply terminals 5 and 6. Also a capacitor 7 is connected between the positive and negative power supply terminals 5 and 6. A motor 8 is connected between the junction of the power MOSFETs 1, 2 and the junction of the power MOSFETs 3, 4. Freewheel diodes 9 to 12 are connected to the power MOSFETs 1 to 4 in parallel, respectively. A control voltage applied to gate terminals 13 to 16 control the power MOSFETs 1 to 4 to turn On or Off.
When the FETs 1 and 4 are in an on-state, a supply current I.sub.1 flows in the motor 8. Then, when the FET 1 turns Off, a circulating current I.sub.2 flows through the freewheel diode 10. Turning the FET 4 Off and turning the FETs 2 and 3 On cause a supply current reverse to the supply current I.sub.1 in direction to flow in the motor 8. When this supply current flows, the free wheel diode 10 enters a reverse recovery stage. Excessive carriers which have been accumulated are swept out and thereby the freewheel diode 10 turns Off.
When the freewheel diodes 9 to 12 require a long reverse recovery time, the resultant heavy switching loss hurts the efficiency. Therefore, in general, external discrete high speed recovery diodes are used as the freewheel diodes 9 to 12 to shorten the reverse recovery time. On the other hand, a MOSFET has the structure having a diode parastically formed in a device, and conventionally the trial for using the diode as a freewheel diode has been made.
FIG. 2 is a sectional view showing a structure of a conventional vertical-type n channel MOSFET. Referring to FIG. 2, an N (or N.sup.-) drift region 22 is formed on an N.sup.+ drain region 21, and P.sup.+ well regions 23 are formed in the surface of the drift region 22. N.sup.+ source regions 24 are formed in the surface of each P.sup.+ well region 23, and regions 25 close to the surface of the P.sup.+ well regions 23 between the surfaces of the N.sup.+ source regions 24 and the surface of the drift region 22 comes to function as channel regions. Gate electrodes 27 are formed on the channel regions 25 through gate insulating films 26. Other insulating films 28 cover the gate electrodes 27 to insulate the gate electrodes 27. Over the entire surface, a source electrode 29 is provided connecting with the N.sup.+ source regions 24 and the P.sup.+ well regions 23. A drain electrode 30 is provided on the bottom surface of the N.sup.+ drain region 21.
In the MOSFET shown in FIG. 2, when a positive voltage is applied to the gate electrodes 27, the channel regions 25 are inverted into an N-type to form inversion layers, and a drain current I.sub.D flows from the drain electrode 30 to the source electrode 29 through the inversion layer as shown in FIG. 2. When a negative voltage is applied to the gate electrodes 27, the inversion layers in the channel regions 25 disappear, and thus the MOSFET turns Off.
The MOSFET shown in FIG. 2 parastically includes a P.sup.+ IN.sup.+ diode 31 formed of the P.sup.+ well region 23, the N drift region 22 and the N.sup.+ drain region 21. Thus, the diode 31 can be used as each of the free wheel diodes 9 to 12 shown in FIG. 1. Symbols I.sub.R in FIG. 2 designates a flow of a circulating current.
FIG. 3 is a sectional view showing a structure of a conventional shorted collector IGBT. This shorted collector IGBT has a structure similar to that of the MOSFET shown in FIG. 2 except that, instead of the N.sup.+ drain region 21 in FIG. 2, P.sup.+ collector regions 32 and N.sup.+ collector shorting regions 33 are alternately disposed on the bottom surface of the N drift region 22. A P.sup.+ IN.sup.+ diode 34 similar to the diode 31 shown in FIG. 2 is parastically formed of the P.sup.+ well region 23, the N drift region 22 and the N.sup.+ collector shorting region 33. Thus, when the collector shorted IGBT shown in FIG. 3 is used instead of the power MOSFETs 1 to 4 in FIG. 1, it is possible that the diode 34 is used as each of the free wheel diodes 9 to 12.
When the diode 31 in FIG. 2 or the diode 34 in FIG. 3 is used as each of the free wheel diodes 9 to 12, there is the advantage that it is unnecessary to separately provide external diodes as the free wheel diodes 9 to 12 and, therefore, such a trial has conventionally been made. However, in order to use the diodes 31 and 34 as free wheel diodes, a period of time required for reverse recovery in the diodes 31 and 34 must be very short. Thus, to shorten the lifetime of excessive minority carriers, a lifetime control like a heavy metal diffusion or an electron ray irradiation must be performed.
Such a lifetime control shortens the lifetime of carriers in the drift layer 22, and causes the increase of a voltage drop (i.e., the increase of an On state voltage) in the drift layer 22 in the On state in the ordinary operation. Therefore, a sufficient optimization is necessary. Moreover, it is known that a lifetime control exerts large influences upon electric characteristics of the device, such as an increase in leak current due to a heavy metal diffusion and a variation in threshold voltage caused by an irradiation of electron rays, other than the increase of the On state voltage, and this must be taken into consideration to perform a lifetime control. Thus, it is very difficult to sufficiently shorten a period of time required for the reverse recovery of the diodes 31 and 34 through the lifetime control with the electric characteristics of the device kept well. Eventually, since a sufficient high-speed can not be obtained, there arises the problem that external free wheel diodes must be added.